File input added to serialized forms caused a change in behavior and only halfway follows spec

Description

According to the spec, file inputs with files attached should be serialized with special values. However, plugins that specialize in uploading files, such as jQuery File Upload, handle serializing file inputs for us and they do it in ways that are able to take advantage of some HTML5 features when available. In other words, I think all we need to do is what we did before 1.9, which was ignore file inputs in serialization.

This test case demonstrates the difference in serialization between 1.8 and 1.9 and shows that the value is still empty string when files are attached.

Ref #12134 where the bug was introduced. Although the spec says type="file" should be serialized, jQuery itself doesn't serialize them so we should leave them out. It should just be a case of adding file to the rsubmitterTypes regex.